1. Field of the Invention
The present invention relates to a liquid crystal display device having columnar spacers, a substrate to be used therefor, and methods for producing such a liquid crystal display device and such a substrate.
2. Description of the Related Art
Liquid crystal display devices are gaining higher performance as the range of their applications becomes broadened. In particular, display modes having broad viewing angle characteristics, e.g., MVA and IPS, have been developed and are receiving further improvements.
In order to improve the display quality of a liquid crystal display device, it is necessary to accurately control the thickness (which may also be referred to as the “cell gap” or “cell thickness”) of the liquid crystal layer, regardless of which display mode is adopted. In particular, in large-sized liquid crystal display devices which have come into use in the recent years, it is necessary to uniformly control the cell gap across a large area.
The cell gap is controlled by spacers which are disposed on a pair of opposing substrates, between which a liquid crystal layer is interposed. Conventionally, fibrous or granular spacers (e.g., plastic beads) are used as spacers, which are disposed by being spread over the substrate. However, this technique of spreading plastic beads cannot control the positions at which the spacers will be disposed, and therefore disturb the orientations of the liquid crystal molecules in each pixel. If the liquid crystal molecule orientations within each pixel are disturbed, the disturbance may be visually recognized as display “coarseness”. Moreover, variations in the cell gap may occur due to the underlying protrusions and depressions of the substrate, thus causing display unevenness.
Therefore, in recent years, it is commonplace to adopt a technique of forming columnar spacers on a substrate through a photolithography process using a photosensitive material. The spacers which are formed by this technique may be referred to as “columnar spacers”, “dot spacers”, or “photo-spacers”.
Referring to FIGS. 6A to 6C, a common method for producing columnar spacers will be described.
First, as shown in FIG. 6A, a photosensitive resin is applied to the substrate 61 by a spin coating technique, for example, thus forming a photosensitive resin layer 63. As the photosensitive resin, a photocurable resin (negative-type photoresist) such as an acrylic resin is used. As light for causing the cure, ultraviolet (UV) is often used.
Next, as shown in FIG. 6B, the photosensitive resin layer 63 is exposed through a photomask 65 having openings (light-transmitting portions) 65a in predetermined positions.
Thereafter, the photosensitive resin layer 63 after the exposure is subjected to development, and by removing the uncured photosensitive resin in the regions which were not irradiated with light, columnar spacers 67 at predetermined positions are obtained as shown in FIG. 6C. If necessary, post-baking at a temperature of about 200° C. to 250° C. is performed.
It is known that not only the positions but the distribution density of the spacers (i.e., how densely the spaces are provided) also affect the display quality. For example, if the distribution density of the columnar spacers is too high, the columnar spacers will not be able to follow the changes in thickness of the liquid crystal layer that occur due to thermal contraction and expansion of the liquid crystal material. Thus, especially at low temperatures, “low-temperature voids” may occur. Moreover, as schematically shown in FIG. 7, when the liquid crystal display device (liquid crystal panel) is placed in a vertical posture, the liquid crystal material will gather toward the lower part of the panel due to gravity, thus causing variations in the gap. Among other problems, this results in unevenness in luminance (hereinafter this phenomenon will be referred to as “lower bulging”) On the other hand, if the distribution density of the columnar spacers is too small, there will not be enough mechanical strength, so that cell gap variations may occur in response to pressing of the display surface (hereinafter referred to as “pressure-induced unevenness”). Therefore, it is imperative that the columnar spacers be provided at an appropriate density.
Therefore, it is described in Japanese Laid-Open Patent Publication No. 9-73088, for example, that the aforementioned problems can be prevented by ensuring that the columnar spacers account for a cross-sectional area of 0.0001 to 0.002 mm2 per mm2. Moreover, Japanese Laid-Open Patent Publication Nos. 11-2718 and 2001-117103 disclose setting the area ratio of the columnar spacers in the range of 0.05% to 1.5% in order to prevent the above problems.
Moreover, Japanese Laid-Open Patent Publication No. 2000-321580 describes that an excellent liquid crystal display device can be provided by setting the area ratio (occupancy) of the columnar spacers in the range of 0.05% to 0.86% and also setting the hardness value (DH=K+Pmax/hmax2, where K is a constant; Pmax is a maximum load; and hmax is a maximum displacement) of the columnar spacers within a predetermined range.
However, the inventors of the present invention conducted studies to find that the aforementioned laid-open patent publications all fail to describe necessary conditions, and, in fact, the aforementioned problems (especially “lower bulging”) cannot be sufficiently suppressed even by producing a liquid crystal display device so as to satisfy the stipulated parameter ranges. For example, in Table 2 (described below) of experimental results, Sample No. 13 is illustrated which has a column area ratio P of 0.00105 and satisfies the conditions stipulated in Japanese Laid-Open Patent Publication Nos. 9-73088 (supra) and 11-2718 (supra). As for Sample No. 23, which is a variant of Sample No. 13 using a material (E) having a spring modulus of 2.40 mN/μm3 as the material of the columnar spacers, the column “lower bulging” reads “×” (i.e., unevenness is recognized even through a neutral-density filter (transmittance: 10%) for cameras).
In particular, in the case of a liquid crystal display device with a complicated configuration, if columnar spacers are formed on a structure including a resin layer (e.g., a color filter layer), it may be difficult to obtain a good display device even if the distribution density of the columnar spacers remains in the predetermined parameter ranges, depending on the physical characteristics (spring modulus) of the resin layer which is in direct or indirect contact with the columnar spacers. Therefore, in order to determine the configuration and distribution density of the columnar spacers for obtaining a good display device, repetitive trial-and-error efforts are required. In order to avoid trial-and-error efforts, it would be necessary to dispose the columnar spacers in a region where there is no resin layer, for example, thus resulting in limitations on the available configuration. Furthermore, the designing methodology for the columnar spacers in the display region and the outlying peripheral region has hitherto been unclear. There have been situations where an inappropriate columnar spacer design in the peripheral region resulted in variations in cell gap around the display region, and hence unevenness in luminance.